1. Field of the Invention
The present invention relates to an objective lens, an optical pick-up device and an optical disk device which are used for recording and reproduction in a high density recording disk such as a DVD and an optical disk such as a compact disk.
2. Description of the Related Art
In an optical disk device, a laser diode for emitting a laser beam having a long wavelength such as an infrared laser beam or a red laser beam has been used. Recently, higher density recording is carried out by using a blue laser beam as compared with the case in which each of the laser beams is used as such JP-A-11-224436, JP-A-2000-123394, JP-A-10-334494.
In a video recorder for recording and reproducing an optical disk corresponding to a blue laser, however, the size of an device itself is very large. For this reason, an optical design can easily be carried out by using means such as a beam expander even if the blue laser is used for an optical system. In addition, also in the case in which recording and reproduction can be carried out over both an optical disk corresponding to the blue laser and an optical disk corresponding to a red laser, there is no particular problem because the device is very large and separate optical systems corresponding to wavelengths can be thus constituted in the device.
In a comparatively thin and small optical disk drive device which can carry out at least one of recording and reproduction over the two optical disks having different wavelengths and is incorporated in electronic equipment such as a notebook computer, however, it is impossible to provide an optical system corresponding to each of the blue laser beam, the infrared laser beam and the red laser beam. In addition, the blue laser beam has a greater spherical aberration than the other laser beams and it is very hard to process the blue laser beam with a common optical system. In addition, the integration of the optical system deteriorates an optical characteristic.
If an inertia center and a thrust center are not designed identically in a conventional optical pick-up actuator moving part, furthermore, an AC tilt is greatly generated when a driving coil is driven in a high frequency region. Consequently, the reliability of the conventional optical pick-up device is deteriorated considerably.
Therefore, the inertia center and the thrust center are designed identically in the conventional optical pick-up actuator moving part, resulting in a reduction in the AC tilt generated on the rolling resonance point of the conventional optical pick-up actuator moving part. However, the inertia center and the thrust center are identical to each other in the conventional optical pick-up actuator moving part. For this reason, the shape of an objective lens holding cylinder in the conventional optical pick-up actuator moving part is limited and the rigidity of the optical pick-up actuator moving part is hindered from being increased.
On the other hand, in a coil for generating a thrust (focus or tracking), a coil effective length is to be decreased in order to adapt a coil driving point position to be a thrust center thereof to the inertia center of the optical pick-up actuator moving part. As a result, the thrust is reduced, that is, a sensitivity is lowered.
In an optical pick-up actuator moving part disclosed in the (JP-A-2001-23202), a mass balance is added and an inertia center and a thrust center are caused to be coincident with each other. The addition of an extra mass balance reduces the rigidity of the optical pick-up actuator moving part, and furthermore, the weight of the optical pick-up actuator moving part is also increased, resulting in a reduction in a thrust.
Moreover, the addition of the mass balance further increases the costs of a component and an assembly.
Further, in the conventional optical pickup device and the conventional optical disk device, a part of a light beam used for recording and reproduction is caused to enter a monitor so as to measure the quantity of light. However, this method is inferior in optical usability and brings about stray light. Additionally, the measurement of the quantity of light has been conventionally performed such that light emitted from an optical unit is measured without changing a light intensity distribution. This method is inferior in signal sensibility and inferior in the accuracy of the writing and readout of information.